Hello Chris, and like most native speakers of English I shared your experience with the sound of rr. The links Rusty provided are indeed helpful. Just to highlight one of their ideas, try to think of it not as "rolling" but rather (as regards the tip of the tongue) "flapping" or as some would say "vibrating," while visualizing the movement of the end of a flag in a high wind. This of course necessitates production of a strong breath of air to make the tip of the tongue vibrate freely. I myself have fallen into the habit of huffing a silent letter h into my rr's, with good results - much better than earlier futile attempts to "roll" the tongue through muscle power alone. Best of luck to you!

Rusty's links are a good place to start. One critical item is that in the middle of a word the spellings "r" and "rr" have different pronunciations.

Your observation that "r" (single letter) sounds a lot like the single-flapped "d" as in the American pronunciation of "butter", "ladder" and "latter" is correct. You might notice that this flap sound in English occurs only when the first syllable is stressed and the second syllable is unstressed. This may account for why you have trouble pronouncing "queréis", because the location of the stressed and unstressed syllables feels backward.